Today I have done leetcode's November LeetCoding Challenge with cpp.

November LeetCoding Challenge 29

Description

Number of 1 Bits

Write a function that takes the binary representation of an unsigned integer and returns the number of '1' bits it has (also known as the Hamming weight).

Note:

• Note that in some languages, such as Java, there is no unsigned integer type. In this case, the input will be given as a signed integer type. It should not affect your implementation, as the integer's internal binary representation is the same, whether it is signed or unsigned.
• In Java, the compiler represents the signed integers using 2's complement notation. Therefore, in Example 3, the input represents the signed integer. -3.

 

Example 1:

Input: n = 00000000000000000000000000001011
Output: 3
Explanation: The input binary string 00000000000000000000000000001011 has a total of three '1' bits.

Example 2:

Input: n = 00000000000000000000000010000000
Output: 1
Explanation: The input binary string 00000000000000000000000010000000 has a total of one '1' bit.

Example 3:

Input: n = 11111111111111111111111111111101
Output: 31
Explanation: The input binary string 11111111111111111111111111111101 has a total of thirty one '1' bits.

 

Constraints:

• The input must be a binary string of length 32.

 

Follow up: If this function is called many times, how would you optimize it?

Solution

const uint32_t m1 = 0x55555555;
const uint32_t m2 = 0x33333333;
const uint32_t m4 = 0x0F0F0F0F;
const uint32_t m8 = 0x00FF00FF;
const uint32_t m16 = 0x0000FFFF;
class Solution { 
public:
  int hammingWeight(uint32_t n) {
    n = (n & m1) + ((n >> 1) & m1);
    n = (n & m2) + ((n >> 2) & m2);
    n = (n & m4) + ((n >> 4) & m4);
    n = (n & m8) + ((n >> 8) & m8);
    n = (n & m16) + ((n >> 16) & m16);
    return n;
  }
};

// Accepted
// 601/601 cases passed (0 ms)
// Your runtime beats 100 % of cpp submissions
// Your memory usage beats 57.22 % of cpp submissions (6.4 MB)